1. Field of the Invention
This invention relates to the testing of electrical power cables and, more particularly, to an improved method and apparatus for determining deterioration of power cable insulation by measuring the dielectric loss of the insulation. The invention provides a technique which permits an accurate and reliable detection of trees in a power cable even if they have not spread throughout the cable.
2. Description of Prior Art
One major cause for aging of power cables that can lead to premature breakdown is heat deterioration. The power cables are also subject to several other forms of deterioration known as water treeing or sulfide treeing. Treeing is the formation of microscopic patterns of weakened material in the cable insulation, and the water trees are considered to develop by penetration of water into the insulation in a tree-like pattern under the influence of an electrical potential. Particularly in case of polyethylene and cross-linked polyethylene insulated cables, moisture and sulfides entrained therein penetrate into the insulation, causing sulfide trees. As trees grow during in-service cable operation, the performance of the insulation deteriorates very quickly or gradually, which eventually causes cable failure. This will lead to consequent blackout and loss of electric power supply to the consumer.
In the prior art, several methods for determining deterioration of cable insulation are known. One method consists of impressing a high DC voltage such as 10 KV onto an inactive cable and measuring the resulting DC leakage current. Another method measures the dielectric loss angle (tan .delta.) of a cable during actual cable operating conditions. There are other methods which superimpose a DC or a low frequency voltage on an active cable to provide a positive indication of deterioration of cable insulation. However, these known approaches have certain inherent drawbacks. For example, the tan .delta. method cannot reliably determine the presence of water trees until their penetration across the entire insulation thickness becomes impending. It is true that the tan .delta. method can detect water trees if they have spread out evenly in the cable. However, there is a general tendency for water trees to develop in localized areas of the insulation rather than over the entire cable length. Accordingly, the tan .delta. method has proved ineffective for reliable and accurate detection of such localized water trees.
It is therefore the usual practice to use such test methods in combination and to comparatively analyze and synthesize the test results for evaluation and diagnosis of insulation deterioration. This will of course increase the reliability and accuracy of tree detection. However, there still remains a need for a straightforward and cost-effective technique for determining deterioration of power cable insulation. The present invention fulfills this need, and further provides related advantages.
It is therefore an object of this invention to provide an improved method and apparatus for determining deterioration of power cable insulation with a view to overcoming the above-stated drawbacks of the prior art.
It is another object of this invention to provide an improved method and apparatus for detecting the presence of water trees, sulfide trees and the like in power cables by measuring the dielectric loss of the cable insulation.
It is a further object of this invention to provide an improved method and apparatus that is capable of detecting penetration-type trees even if they have not spread evenly throughout the power cable.
It is a still further object of this invention to provide an improved method and apparatus which can detect the presence of trees with high reliability and accuracy but which is relatively simple and cost effective to implement.